Organic liquid waste

The volume of organic waste is small by comparison with aqueous radioactive waste, however, the risk associated with its improper management may be high. Aqueous waste may be discharged to the environment after the radioactivity has decayed or been removed by treatment.

By contrast, organic radioactive waste requires management steps that not only take account of its radioactivity, but also of the chemical organic content since both can have detrimental effects on health and the environment. The "dilute and disperse" option open for some aqueous and gaseous waste is not appropriate for most of organic liquid waste.

The goals of organic liquid waste treatment may be as follows:

· conversion to a solid form;

· conversion to an inorganic form to facilitate conditioning;

· volume reduction;

· decontamination for reuse;

· conversion to an organic form compatible with cementation.

The main features of treatment methods OF organic liquid waste have been summarized in Table VII.

TABLE VI. MAIN FEATURES OF THE AQUEOUS WASTE TREATMENT PROCESSES

· Suitable for large volumes and high salt content waste

· Easy industrial operations

· Not expensive

· Generally lower DF than other processes (10 < DF < 10²)

· Efficiency depends on solid-liquid separation step

Organic ion

exchange · DF good on low salt content

· (10²)

· Good mechanical strength

· Regenerable

· Limited radiation, thermal and chemical stability

· Resins cost important

· Immobilization difficulty Inorganic ion

exchange · Chemical, thermal and radiation stability better than organic ion exchangers

· Relatively easy immobilization

· Large choice of products ensuring high selectivity

· DF > 10 to 10⁴

· Affected by high salt content

· Blockage problems

· Possible high cost

· Regeneration and recycling often difficult

Evaporation · DF > 10⁴ to 10⁶

· Well established technology

· High volume reduction factor

· Suitable for a large number of radionuclides

· Process limitations (scaling, foaming, corrosion, volatility of certain radionuclides )

· High operation costs

· High capital costs

Reverse osmosis · Removes dissolved salts

· DF 10²–10³

· Economical

· Established for large scale operations

· High pressure system, limited by osmotic pressure

· Non-backwashable, subject to fouling

Ultrafiltration · Separation of dissolved salts from particulate and colloidal materials

· Good chemical and radiation stability for inorganic membranes

· Pressure <1MPa

· Fouling-need for chemical cleaning and backflushing

· Organic membranes subject to radiation damage

Microfiltration · Low pressure operation (100–150 kPa)

· High recovery (99%)

· Excellent pretreatment stage

· Low fouling when air backwash employed

· Backwash frequency can be high;

depends on solid content of waste stream

Electrochemical · Low energy consumption

· Enhances the effectiveness of reactions · Sensitive to impurities in waste stream

· Ionic strength of waste stream can effect performance

· Fouling is a problem above 10 g/L total solids

Solvent extraction · Selectivity enables removal, recovery or

recycle of actinides · Organic material present in aqueous raffinate

· Generates aqueous and organic secondary waste

Because individual waste generators may not have expertise in the management of organic waste, and as the treatment of small quantities of organic waste may not be cost effective, it may be appropriate to consider the transportation of liquid organic waste to a central waste management facility where the necessary expertise, infrastructure and quality assurance capability can be built up.

A general flow sheet for managing organic liquid waste is shown in Fig. 7. As can be seen from a comparison of this flow sheet with the flow sheet for managing aqueous waste, treatment of large amounts of radioactive liquid organics is technology intensive as well as costly. Here is a clear opportunity for a Member State that is considering technology involving this waste as a by-product to rethink the total life cycle cost of that technology and perhaps choose an alternative.

Properly controlled incineration is an attractive technique for treating organic liquids because they are readily combustible, and high volume reduction factors can be achieved.

Disadvantages inherent in using this technology are the high capital cost of the incinerator and the off-gas system, and the technical expertise required to operate and maintain the unit. Ensuring complete combustion of the waste and maintaining stack emissions within acceptable limits (which may be carefully detailed as a part of the operating licence) are the main technical difficulties for waste incineration. In addition to containing volatile radionuclides and radioactive particulates, the off-gas system must control the release of chemically toxic or noxious effluents (HCl, SO₂, NO_x). Improperly controlled combustion can produce toxic compounds, like dioxins.

After combustion, radionuclides from the waste will be distributed between the ash, filters and off-gas, depending on details of the unit's design and operating parameters. Further immobilization, such as grouting, will be required to stabilize these residues which will now have much higher radionuclide concentrations per unit volume than the original waste.

TABLE VII. MAIN FEATURES OF LIQUID ORGANIC WASTE TREATMENT METHODS

Treatment methods Features Limitations

Incineration · Decomposes organic nature of waste

· High volume reduction

· Combined use for other waste

· Eliminates infectious hazard

· Secondary waste must be treated

· High temperatures are required to ensure complete decomposition

· Off-gas filtration and monitoring are required

Emulsification · Allows embedding of liquid organic waste into cement matrixes

· Low limitations for content of emulsified liquids in the cement matrix

Absorption · Solidifies and immobilizes organic liquids

· Simple and cheap

· Suitable only for small amounts of waste

· Absorbed waste may not meet disposal acceptance criteria

Phase separation

(e.g. distillation) · Removes water and detoxifies the

waste for direct disposal · Non-universal application.

· Technology is relatively expensive for this type of waste

· Produce clean solvent Wet oxidation · Low temperature process

· Simpler than incineration

· Suitable for biological waste

· Requires storage of oxidizing agent

· Residue requires immobilization

FIG. 7. Management flow diagram for liquid organic waste.

In many cases, processes selected for the treatment of aqueous waste can be adapted to the processing of organic liquid waste and combined processing could be cost effective. Where dedicated equipment for the destruction of organic liquid waste is desired, equipment cost, versatility for the treatment of a number of organic liquid waste, equipment availability, reliability in operation, and ease of maintenance are all factors to be considered in the selection of the process. Often, substantial advantages can be accrued by selecting a combination of two or more processes, rather than a single process.

A simple way of on-site treatment of organic liquid radioactive waste is converting the liquid to a solid form with absorbents. As long as there is an excess of absorbent there is no need even for mixing; the liquid waste can be added to the absorbent in a suitable container and eventually all the liquid will be taken up. This technique has been routinely used for the solidification of radioactive turbine and pump oil. The following main categories of absorbent are commonly used:

· Natural fiber (sawdust, cotton).

· Synthetic fiber (polypropylene).

· Vermiculite (mica).

· Clays.

· Diatomaceous earth.

The use of absorbents converts the liquid waste into a form which can vary from loose dry particles to a jelly-like solid. The waste forms have no special integrity and are only restrained from dispersing by the container. The absorption efficiency of the different absorbents can vary by a factor of 2 to 3, and the waste volume increase can be up to almost 300% [19].

The suitability of absorption alone for the solidification of organic liquid waste is only moderate; the process efficiency can be adversely affected by the presence of water or other ionic contaminants, and variations in waste viscosity can cause significant reductions in the quantity of liquid absorbed. The waste form is readily dispersible in air or water if the product container is breached. Finally, the oil released from these waste forms following the disintegration of the drums can directly affect the radionuclide retention prospectus of buffer and rock in a disposal facility. This may be important particularly when co-disposal of these waste forms with other radioactive waste is considered.